Electrical fuse

ABSTRACT

The present invention relates to an electrical fuse element ( 1 ) which comprises at least one fusible conductor ( 3 ) and a carrier ( 2 ). The object is to provide a fuse element ( 1 ) for all known tripping characteristics in a cost-effective production technique for the middle and low-current range. Furthermore, by means of a small outer geometry, the fuse element ( 1 ) is to be adaptable to modem methods of insertion. The way in which this object is achieved according to the invention is that the carrier ( 2 ) consists of a material of poor thermal conduction, in particular of a glass ceramic.

DESCRIPTION

The present invention relates to an electrical fuse element according tothe preamble of claim 1.

Fuse elements are used in large numbers for protecting electrical andelectronic circuits from excessive currents. In such cases, they have tobe adapted to the current ranges occurring in an application, by thetripping characteristics respectively required. The generallyperceivable and ever increasing tendency for circuit components to bemade smaller while having the same or even enhanced capabilities leadsto considerable problems in the area of electrical fuse elements.

EP 0 515 037 A1 discloses a fuse located on the substrate of a hybridcircuit, where the fuse is supported on a thermally insulating layer andteaches to adjust the operating parameters of the fuse by varying e.g.the degree of thermal insulation about the fusible track. With using asupport layer having a high thermal resistivity, the effect of raisingthe total thermal resistivity is not achievable. Thus a fuse cannot bereduced in size. Further very few fusing characteristics can be realizedin this way only.

It is therefore the object of the present invention to provide a fuseelement for all known tripping characteristics by a cost-effectiveproduction technique for the medium and low currant range. Furthermore,by having a smaller outer geometry, the fuse element is to be adaptableto modern methods of insertion.

According to the invention, this object is achieved by that

the carrier consists of a material of poor thermal conduction, inparticular of a glass ceramic,

the fusible conductor is indirectly heated, preferably by at least oneadditional heating element, where

at least one heating element is arranged together with the fusibleconductor jointly on the substrate and

a distance between the heating element and the fusible conductor isvariable, in order to set the degree of thermal coupling with otherwisethe same geometry of the circuit.

In the past there have been numerous known attempts to make the outerdimensions of electrical fuse elements considerably smaller whileretaining their operational current range, their switching capacity andtheir specific tripping characteristics. However, these attemptsresulted in failure because either the internal heating of the fuseelement became too great and/or the desired tripping characteristiccould not be achieved, or the fuse element became unsoldered at itscontact points owing to increased self-heating.

By the use according to the invention of a carrier made of a material ofpoor thermal conduction, the present invention overcomes a widespreadprejudice to the use of such materials in fuse construction. By the useof such a carrier material, the hot zone (hot spot) of the fuse elementcan be advantageously restricted to the core region of the carrier or ofthe housing, since the heat dissipation is very low. Thus, the heatremoval by conduction via the external contacts is significantly less.Consequently, unsoldering of its own accord or inadmissible heating isno longer possible for a fuse element according to the invention.Furthermore, by concentrating the “hot spot” in a certain region, theentire power consumption of a fuse element according to the invention islowered. Thus, a minimal power consumption also results in less of aretroactive effect on the surrounding electric circuit.

Among suitable materials of poor thermal conduction are ceramics, glassceramics or glass. Glass ceramics are preferred, however.

For a cost-effective mass production of fuse elements according to theinvention with their small geometrical sizes, formation of the carrierin sheet form is advantageous, preferably in the form of a sheet-likesubstrate. Thus, fuse elements according to the invention can beproduced in a cost-saving manner in multiple repeats, for example in thesize of customary service-mounted devices (SMDs) on a planar substrate.

In a fuse element according to the invention, the fusible conductor mayact as a single heat source. However, to set different nominal currentsand switching characteristics, an indirect heating of the fusibleconductor is preferred.

At least one additional heating element serves for this purpose. In thecase of some embodiments, two heating elements are used with preference,for example, as is shown below with reference to illustrations of anumber of exemplary embodiments according to the invention. Cases withmore than two heating elements are also conceivable. When reference ismade below to a heating element, these possibilities are also intendedto be included.

A fuse having an addition a heating element is known e.g. form AT-B 383697. The fusible element is thermally coupled to a resistor where thecomponents are located on the same sheet-like substrate. The substrateis made of a ceramic material. The resistor acts as a current sensor. Anamount of thermal energy sent from the resistor to the fusible elementis equivalent to the amount of current. But within the teaching of thisdocument there is no way of changing the characteristics of the fuse.

According to the invention, the heating element is arranged togetherwith the fusible conductor jointly on the substrate. In this case, thedegree of thermal coupling between heating element and fusible conductoris influenced in each case by the distance from each other. Theconsequently achievable effects of shifting the characteristic curve ofthe fusible element are explained in more detail below with reference toexemplary embodiments.

Further the distance produced between the heating element and thefusible conductor is kept variable, in order to set the degree ofthermal coupling and consequently the tripping characteristic of thefusible conductor and the nominal current while otherwise retaining thesame materials and the same geometry of the circuit. With a fixedcircuit geometry, setting of the characteristic is possible by simplyshifting the individual production masks in relation to one another in apredetermined way and fixed amount.

In a development according to claim 2 the distance between the heatingelement and the fusible conductor assumes a minimal value when theheating element and the fusible conductor are arranged lying one overthe other. This minimal value is in this case determined by the layerthickness of an electrical insulation, which may consist of a dielectricsuch as glass, but also a ceramic or a curable paste. The good thermalcontact may take place over the entire base area of the fusibleconductor. Preferably, the fusible conductor is arranged over theheating element, so that there is adequate space available for receivingthe gases and particles released in the event of the fusible conductortripping, as well as for pressure equalization.

According to the invention, the properties of the fusible conductor canbe significantly influenced directly by the thermal coupling with theheating element. The thermal coupling is intensified in a simple way bythe actual fusible conductor being applied to a thin layer, whichpreferably consists of silver and effects adhesive bonding with goodconduction on the substrate surface. As a result, the characteristic canbe reproduced even more exactly.

In the case of a fusible conductor formed as a multilayer arrangement,for example in the case of a material combination of a layer of silverand a covering layer of tin, an additional influencing of the trippingcharacteristic can be achieved by diffusion processes. Other materialcombinations with mutual solubility are also possible.

Furthermore, the fusible conductor may have a constriction or taperingin its central region. This reduction in cross-section increases theintrinsic resistance. What is more, the material of the fusibleconductor is weakened at this notable point and correspondingly lessmaterial has to be melted during tripping. The constriction isadvantageously in the “hot spot” of the fuse element.

Alternatively, the fusible conductor may, however, also be a wire, whichhas, for example, as described above, a silver-tin layering on itssurface and/or itself a constriction. To improve the thermal coupling,the wire may be pressed onto or fused onto the substrate.

There are, in principle, several possibilities that are conceivable forthe electrical wiring to supply the heating element and the fusibleconductor with power, for example a parallel connection. However, it ispreferred for the heating element to be electrically connected in serieswith the fusible conductor on the substrate. Consequently, with the insome cases very small outer dimensions, only two external contacts arerequired on a fuse element according to the invention.

In a major development of the invention, the heating element itself isalso designed as a fusible conductor. This provides a fuse elementaccording to the invention as an electrical connection of two fuseelements, which are in their design primarily assigned the tasks ofheating element and fusible conductor by the selection of material andgeometry. This type of construction advantageously opens up thepossibility of designing the heating element for a different, preferablymuch higher nominal current I_(N) than the fusible conductor. Bydesigning the characteristics of the fusible conductor and heatingelement in the way according to the invention, these curves intersect ata commutation point. From this point, the fusible conductorcharacteristic of the heating element responds faster than the actualfusible conductor, as will be shown with reference to a diagram. For thefollowing electric circuit, this produces additional protection in thecase of extremely high short-circuit currents.

A further advantage is obtained by a covering, preferably of eachfusible conductor, by means of a low-melting substance. In the event oftripping of the fuse, the covering prevents molten parts coming intocontact with the surroundings. It may be realized in the form of atwo-layer structure, a drop of hot-melt adhesive as the core, forexample, being covered for its part on the outside and sealed by athermally stable substance, such as for example a curing embeddingcompound or a resin. At operating temperature, the core already meltsand creates a cavity for receiving gases etc., which is stabilized bythe outer shell.

Advantageously, an electrical fuse element according to the inventioncan be easily adapted in its outer form and dimensions to therequirements of modern insertion methods. A cuboidal form is preferred.The external contacting takes place in adaptation to customary SMDsoldering methods by external contacts arranged on two opposite endedges. They are then preferably applied in a galvanic process, iffusible elements with diffusion processes are contained in the fuseelement.

A number of exemplary embodiments of the invention are explained in moredetail below with reference to the drawing, in which:

FIG. 1a shows a basic representation of a first embodiment of a fuseelement in a plan view;

FIG. 1b shows a representation of an alternative embodiment of the fuseelement from FIG. 1a;

FIG. 1c shows a representation of a further alternative embodiment ofthe fuse element from FIG. 1a;

FIG. 2 hows a plan view of a further embodiment of a fuse element with afusible conductor arranged over the heating element;

FIG. 3 shows a perspective view of a fuse element in an explodedrepresentation and

FIG. 4 shows a sketched family of characteristic curves with theswitching characteristics achievable in principle of the fuse elementsfrom FIG. 1c and FIG. 2.

In FIG. 1a, a first embodiment of a fuse element 1 is represented in itsbasic structure in a plan view. A fusible conductor 3 is arrangedtogether with two heating elements 4 in an S-shaped series connection ona substrate 2 of poor thermal conduction. The individual elements areelectrically connected to one another by conducting tracks 5. There isthus obtained here overall a series connection of three elements, whichmay in each case be designed as a fusible conductor with specificproperties. The two heating elements 4 are arranged here symmetricallywith respect to the fusible conductor 3 at a distance d, which in bothcases is equal. Thus, they heat up the fusible conductor 3 by thermalconduction via the substrate 2 equally in a symmetrically shaped “hotspot”.

Among the materials used for substrate 2 of poor thermal conduction is aglass ceramic. Measurements have produced the following, surprisingvalues for the thermal conductivity of such a material in comparisonwith the Al₂O₃ ceramic otherwise preferred in fuse construction:

Static thermal Substrate resistance Thermal impedance Glass ceramic 190K/W   6 K/W Al₂O₃ ceramic  26 K/W 5.4 K/W

It is evident from the values in this table that an Al₂O₃ ceramicdissipates the heat per watt of heating output between the ends of asubstrate better by a factor of approximately 7 than the glass ceramicmeasured here. These values relate to the consideration of the case ofsteady-state heat removal, which in the case of Al₂O₃ ceramic substratesleads to the undesired unsoldering of the external contacts.

If, however, the investigation is restricted to the dynamic thermalconduction behaviour and if, correspondingly, a very small space isconsidered, also referred to as a segment, only a relativelyinsignificant difference in heat removal of about 10% is establishedbetween Al₂O₃ ceramic and the glass ceramic. The thermal couplingbetween fusible conductor and heating element is thus almost as goodwith the use of a glass ceramic substrate as in the case of an Al₂O₃ceramic substrate. Accordingly, significant differences occur only inthe consideration of the thermal conduction at the ends of commonsubstrate sizes, where an A1 ₂O₃ ceramic effects an undesired heating ofthe external contacts on account of its much better thermal conduction.

The degree of thermal coupling between the heating element and thefusible conductor can be set over a wide range by the distance d. Theinfluence of the thermal coupling on the switching characteristics ofthe fuse element is shown and described later with reference to a familyof characteristic curves.

Arranged adjacent to two opposite end edges 7 of the substrate 2 areconducting faces 8. To complete the production process, the end edges 7are metallized, so that they form the external contacts 9, which areelectrically connected to the faces 8. Use of the substrate 2 of poorthermal conduction has the effect that there is little heating up of theexternal contacts 9. There is consequently also a reduction in the powerloss of the fuse element required as heating power, so that this fuseelement 1 has little influence on the remaining electric circuit.

The fuse element 1 from FIG. 1a has been realized in its essential partsby a screen-printing process. In the case of very small structure sizes,a photolithographic process is more suitable. In the present case, thefusible conductor 3 is produced as a thick film, which has a tapering 6in its central region. The tapering 6 is a further measure forinfluencing the tripping characteristic. Depending on the desiredcharacteristic, it may also be omitted. As a further productionpossibility, the fusible conductor 3 may also be used in the productionprocess in the form of a piece of wire. In the present case, the fusibleconductor 3 is applied to the substrate 2 as a thin layer of silver,onto which subsequently a layer of tin is applied as the actual,low-impedance conductor.

The central region of the fuse element 1, in which the heating elements4 and, in particular, the fusible conductor 3 are located, is providedwith a covering 10. The covering 10 is indicated in FIG. 1a as a dashedline and protects the sensitive part of the circuit on the substrate 2from external influences. Furthermore, gases or metal particles emittedduring tripping of the fuse element 1 are kept away from the surroundingelectric circuit.

FIG. 1b represents an alternative form of the fuse element 1 from FIG.1a, which contains only a heating element 4 and a fusible conductor 3without constriction 6. The thermal coupling entered in the form ofarrows, is less than in the arrangement from FIG. 1a on account of theappreciably increased distance d between heating element 4 and fusibleconductor 3. The basic representation of FIG. 1b is primarily intendedto demonstrate the freedom of design, with several possibilities for thearrangement, although no change has been made to the basic geometry ofthe circuit, comprising conductive faces 8, external contacts 9 andconductive tracks 5.

FIG. 1c represents a further developed form of the fuse element 1 fromFIGS. 1a and 1 b, in which the heating element 4 and the fusibleconductor 3 are again moved closer together, reducing the distance d, toincrease the thermal coupling. It is intended by the different type ofrepresentation in FIG. 1c to point out that the regions of the faces 8and conductive tracks 5 of good electrical conductance can also beproduced in two or more mask steps. Setting the thermal coupling byvariation of the distance d is advisable, however, when using two masksfor building up the conductive tracks 5 and 5 a, since in this way thedistance d can easily be changed by shifting the masks in relation toeach other, without the production of a new mask being required.

FIG. 2 represents a plan view of an alternative embodiment of a fuseelement 1, the fusible conductor 3 here being arranged over the heatingelement 4 on the substrate 2. Arranged between the fusible conductor 3and the heating element 4 is an electrical insulation 11, which isformed here by way of example by a thin layer of glass. The thermalcoupling in the embodiment represented takes place over the entiresurface area of the fusible conductor 3 and therefore, and because ofthe minimal distance d_(min), increases to a maximum value.

Depending on the selection of materials, the circuit from FIG. 2 mayalso be produced in two process steps, which are in each case completedby a sintering operation. In a first step, the conducting faces 8, theconductive tracks 5, the heating element 4 and the insulation 11 overthe heating element are applied in one mask. In a subsequent productionstep, the second level is applied, which essentially contains thefusible conductor 3 and two conductive tracks 5, which electricallyconnect a conducting face 8 to the fusible conductor and establish aconducting connection with the lower level of the circuit via acontacting assembly 12.

Subsequently, the circuit may be covered, at least in the region of thefusible conductor 3, by a curing embedding compound. This covering isapplied in two steps, with a low-melting substance being applied firstof all. This is, for example, a hot-melt adhesive, which covers only thefusible conductor. It is covered by a thermally stable substance. Duringthe operation of the fuse element, the melting drop of adhesive createsdirectly above the fusible conductor, in the “hot spot”, a stable cavityfor receiving plasma during the tripping of the fuse element 1.

The direct comparison of FIGS. 1c and 2 shows that, in principle, thesame masks are used here for producing fuse elements with very differentswitching characteristics and/or nominal currents I_(N). Introducing theinsulation only necessitates one further mask step in the productionsequence according to FIG. 2. The mask of the upper conductive track 5 arequires a small modification. Essentially, however, these structuresare the same as one another. Consequently, only one set of masks isrequired for producing a wide variety of SMD-insertable fuse elementsand a standard, adapted range of pastes or the like can be used incost-effective mass processes.

FIG. 3 perspectively shows in an explosive representation a design for afuse element 1 with all the individual elements listed above. The solidlines and arrows in this case represent conducting connections. The line13 shows the outline of the bearing face for the insulation 11. Theelements represented in planes may be produced here as layers, in eachcase by a process mask. The arrangement of the elements with respect toone another and the forming of the conductive tracks 5 opens up thepossibility here that the fusible conductor 3 and the heating element 4can be varied in relation to each other by shifting the process masks interms of the distance d between them. The variation in distance is notshown in this illustration. However, the arrangement represented in FIG.3 can be used correspondingly to realize, as limiting cases, either fuseelements according to FIG. 2 or fuse elements according to FIG. 1c. Inthis case, the fuse element 1 according to FIG. 2 contains only oneheating element 4, so that, although the thermal coupling can be sethere by variation of the distance d, the “hot spot” is not fullysymmetrically formed in the region of the fusible conductor 3. However,this influence can be minimized by appropriate design of the circuit. Assoon as the distance between the tapering 6 of the fusible conductor 3and the heating element 4 is large enough that there is no overlapbetween fusible conductor 3 and heating element 4 and an adequateinsulation between the conductors is obtained, the insulation 11 may beomitted, thus dispensing with one substep in the process.

FIG. 4 represents a sketched general family of characteristic curves torepresent switching characteristics of different fuses. The curves areplotted with a logarithmic scale on both axes. It can be seen that, inthe present case, the heating element alone is designed for a lowernominal current I_(N) than the fusible conductor. The fusible conductoris, for example, built up as a multilayer conductor by using asilver-tin diffusion and accordingly has only a quick-acting switchingcharacteristic, while the heating element alone trips with a very quickaction. With this design of the individual elements, the seriesconnection with thermal coupling allows an increase in the inertia inthe overall fuse element to be achieved. In the converse case, a greatertripping capacity can be produced.

The characteristic of the individual elements in any event differsdistinctly from that of the overall circuit. It shows here a distinctlyslow-acting characteristic, which until now could not be realized bycomponents of small dimensions. The influence of the thermal couplingbetween the heating element and the fusible conductor can be seen in theshift to the left, into the range of lower nominal currents I_(N), ofthe curve for the switching characteristic of the fusible conductor. Thecurve in itself changes its shape only insignificantly. By variation ofthe distance d, the shifting of the fusible conductor characteristic canbe influenced. With a minimal distance d_(min), the nominal currentI_(N) assumes a minimal value if the material and the geometry of thefusible conductor remain the same, see curve B. Consequently, by aconstruction according to FIG. 3, the wide range between the curves Aand B represented in FIG. 4 can be freely set during production byvariation of the distance d. Consequently, with the geometry andmaterial selection remaining the same, a large range of nominal currentscan be covered with the same tripping characteristic.

In the lower third, the shifted curves intersect with the characteristicof the heating element at a so-called commutation point K. This point isin practice to correspond to a current of slightly more than 10×I_(N).For higher currents, the curve of the heating element then determinesthe tripping characteristic of the respective fuse element, no longerthe characteristic of the indirectly heated fusible conductor. Thus,faster tripping times are realized for higher short-circuit currents.

In tests, fuse elements were constructed with substrate dimensions of6.5×25 mm and 46×3.2 mm. These are common dimensions in SMD technology.At ten times the nominal current I_(N), switching times of 10-15 ms weremeasured for nominal currents of about 0.4 A. Consequently, efficientfuse elements with slow-acting tripping characteristics were realizedfor the first time in the size of SMD components. With a fuse elementcorresponding to FIG. 1c, the heating resistance was 0.6 Ω. The fusibleconductor resistance was in this case 0.03 Ω. Thus, for the seriesconnection, altogether only a resistance of about 0.63 Ω is obtained.

In the case of the variant according to FIG. 2, a heating resistance of0.1 Ω and a fusible conductor resistance of 0.03 Ω were realized for anominal current I_(N) of about 0.315 A, a layer of glass of thethickness d_(min) of about 20 μm being used as the dielectric. Bothcircuit variants were produced by thick-film technology on a glassceramic substrate, using paste materials common in hybrid technology. Inthick-film technology production processes, currently line widths of upto 0.1 mm can be reliably produced in the case of layer thicknesses ofbetween 6 and 20 μm.

It can be seen from these actually realized exemplary embodiments that,in the case of the variant according to FIG. 2, the heating resistanceof the heating element 4 may turn out to be relatively low on account ofthe much improved thermal coupling.

What is claimed is:
 1. A surface mounted electrical fuse element,comprising a sheet-like substrate including a glass ceramic, thesubstrate having a top and a bottom surface; at least one fusibleconductor; a resistive element acting as a heating element, said atleast one fusible conductor and said resistive element being arranged onthe top surface of the substrate and in thermal contact with each other,and said at least one fusible conductor and said resistive elementforming a series circuit; and wherein when a current flows through theseries circuit formed by the at least one fusible conductor and theresistive element, the resistive element heats the at least one fusibleconductor such that the fuse element exhibits a slow-acting trippingcharacteristic.
 2. Electrical fuse element according to claim 1, whereinthe resistive element is also designed as a fusible conductor. 3.Electrical fuse element according to claim 2, wherein the resistiveelement is designed for a defferent nominal current than the at leastone fusible conductor.
 4. Electrical fuse element according to claim 1,wherein a distance between the resistive element and the at least onefusible conductor assumes a minimal value when the resistive element andthe at least one fusible conductor are arranged lying one over the otherseparated by an insulating layer or insulation.
 5. Electrical fuseelement according to claim 1, wherein the at least one fusible conductoris formed as a multilayer arrangement.
 6. Electrical fuse elementaccording to claim 1, wherein the at least one fusible conductor has aconstriction.
 7. Electrical fuse element according to claim 1, whereinthe at least one fusible conductor is a wire.
 8. Electrical fuse elementaccording to claim 1, further comprising a cover above each fusibleconductor, said cover includes a low-melting substance covered by athermally stable substance.
 9. Electrical fuse element according toclaim 2, wherein the at least one fusible conductor is formed as amultilayer arrangement.
 10. Electrical fuse element according to claim2, wherein the at least one fusible conductor has a constriction. 11.Electrical fuse element according to claim 2, wherein each fusibleconductor is covered by a low-melting substance, and the low-meltingsubstance is covered by a thermally stable substance.
 12. Electricalfuse element according to claim 1 wherein the resistive element isdesigned for a higher nominal current than the at least fusibleconductor.
 13. Electrical fuse element according to claim 9 wherein saidmultilayer arrangement includes a layer of silver and a covering layerof tin.
 14. Electrical fuse element according to claim 11 wherein saidlow-melting substance is a hot-melt adhesive.
 15. Electrical fuseelement according to claim 11 wherein said thermally stable substanceincludes a curing embedding compound or a resin.
 16. Electrical fuseelement according to claim 5 wherein said multilayer arrangementincludes a layer of silver and a covering layer of tin.
 17. Electricalfuse element according to claim 8 wherein said low-melting substance isa hot-melt adhesive.
 18. Electrical fuse element according to claim 8wherein said thermally stable substance includes a curing embeddingcompound or a resin.
 19. Electrical fuse element according to claim 1wherein said substrate consists of glass ceramics.
 20. A surface mountedelectrical fuse element, comprising a sheet-like substrate consisting ofa material having a static thermal resistance that is approximatelyseven times greater than that of Al₂O₃ for a thermal impedance, thesubstrate having a top and a bottom surface; at least one fusibleconductor; a resistive element acting as a heating element, said atleast one fusible conductor and said resistive element being arranged onthe top surface of the substrate and in thermal contact with each other,and said at least one fusible conductor and said resistive elementforming a series circuit; and wherein when a current flows through theseries circuit formed by the at least one fusible conductor and theresistive element, the resistive element heats the at least one fusibleconductor such that the whole fuse element exhibits a slow-actingtripping characteristic.